1. Field of Invention
This invention relates to techniques of manufacturing a rigid-flexible printed circuit board (rigid-flex PCB), and more particularly to form a reliable peelable mask on a flexible circuit substrate to protect conductive patterns, such as conductive traces, from damage while processing.
2. Related Prior Art
Taiwan utility model Patent No. M259439 discloses a protect layer printed on conductive traces of a (rigid) printed circuit board. The protect layer is made of silicone-based paste, which may be cured under ultra-violet light. The protect layer is elastic with a thickness about 0.1 to 0.2 mm and is bearable under a temperature up to 250□ such that the protect layer may endure through an electroplating process or a reflow process and then be peeled away. However, a rigid-flex PCB is substantially more difficult to manufacture than the rigid one, due to its fragile characteristics. Accordingly, the aforementioned protect layer which is usually subject to severe conditions may not be suitable for use in the rigid-flex PCB.
As disclosed in the background section of U.S. Pat. No. 7,281,328, a conventional process of fabricating a rigid-flex PCB is disclosed, including the step of forming a resist cover on a part of a flexible region of the rigid-flex PCB. The resist cover functions to protect the internal circuit pattern which is to be exposed for use in an external pad or a mounting pad. In the case where the resist cover is formed of peelable ink, the peelable ink may be easily removed by further applying peelable ink on the formed peelable ink to form an ink layer having a predetermined thickness and then removing the ink layer. However, under harsh chemical and physical conditions, residue may remain after removal of the peelable resist cover, contamination may occur, and therefore a defect rate is increased, resulting in drastically decreased reliability.